Aviation-assisted devices for the determination of distance by means of light pulses are used particularly for the measurement of data before the establishment of digital terrain models, where the distance to the site of the reflection is determined from the travel time of the light pulse from the device to the site of the reflection and again back to the device (i.e., the light echo).
Such devices and methods are known to the person skilled in the art by the term LIDAR (light detection and ranging).
Conventional LIDAR devices work with a so-called “single pulse in air” method. In such a method, there is always only one light or laser pulse on the path between the light source of the device, the site of the reflection, and the detector of the device. In the process, the problem that arises is that with increasing distance of the aviation-supported LIDAR device from the site of the reflection, the maximum light pulse frequency, i.e., the frequency at which the light pulses can be emitted by the LIDAR device, decreases necessarily, because the travel time of the light pulses increases with increasing distance. It is clear that it is advantageous to work at a high light pulse frequency, because it allows, in comparison to a lower light pulse frequency, a finer scanning (higher data point density), and thus a better resolution of the terrain to be measured.
The dependency of the maximum possible light pulse frequency fmax on the flying altitude, without taking into consideration a system-related temporal offset in the order of magnitude of 10−6 sec, results from the following relation:fmax=c/(2×h),where c is the speed of light, and h is the flying altitude. Using the above relation, one gets, for example, for a flying altitude h of 1500, 3000 and 4500 m, a maximum light pulse frequency of 100, 50 and 33 kHz (respectively 91, 48 and 32 kHz taking into account a system-related temporal offset).
The consequences of the above-described effect, namely that, in conventional LIDAR devices, the maximum possible light pulse frequency decreases with increasing flying altitude, if the LIDAR device is designed, for example, as a fiber optic scanner in which the terrain is scanned transversely to the flight direction, are reinforced by the fact that, at constant angle of beam and with increasing flying altitude, the separation between two adjacent measurement points in the object plane is increased, which effectively leads to a further decrease in the measuring point density.
In the German Patent DE 36 06 544 C1, a device for measuring distance by laser according to the preamble of Claim 1 is described with a transmitter and a receiver, where the pulse travel time represents a measure of the distance. To provide an interference-insensitive start trigger signal generation that allows the determination of a precise trigger time point, one provides, in the immediate vicinity of the detector element provided for the remote signal, at least one additional detector element which reacts to radiation that is indeed transmitted by the transmitter, but does not reach the target reflection, being instead scattered in the housing.
U.S. Pat. No. 4,926,185 describes a pulse radar system in which two or more consecutive transmission pulses are transmitted cyclically with different carrier frequencies, allowing an unequivocal temporal assignment of the given reception pulse to the transmission pulse.
The present invention is thus based on the problem of providing an improved device or an improved method for the determination of distance by means of light pulses, which avoids particularly the above-mentioned disadvantages of known LIDAR devices and LIDAR methods.